This invention relates generally to gas turbine engines and, more particularly, to a method and apparatus for the control of thrust in turbofan engines.
Turbofan engines are required to operate efficiently over a wide range of flight conditions and speeds. These conditions include maximum power takeoff and climb, part-power cruise, and low-altitude, low-power loiter. In order to meet these requirements of variable thrust while maintaining an acceptable level of specific fuel consumption, it is common to selectively vary certain flow areas and characteristics within the fan and core portion of the engine. One of the flows which may be varied is that in the fan duct, and this many be accomplished by use of variable inlet guide vanes (IGV's) which are selectively varied over a range to modulate the total airflow in the duct.
In a vertical takeoff and landing (VTOL) type aircraft, there are further requirements for the propulsion system. In addition to providing a total lift greater than the maximum gross takeoff weight, the system must also provide the moment necessary for pitch, yaw and roll control of the aircraft. In the vertical mode of operation, this control moment must be supplied entirely by the propulsion system since the aircraft is essentially at zero flight speed and the conventional control surfaces are unable to assist in this control. For example, in a two-engine aircraft, in order to provide the necessary moment for aircraft attitude control during vertical takeoff and landing, the engines must be capable of operating at a high response rate to modulate the thrust in the range of +/-20%. Further, in order that the aircraft retain its altitude while executing the various control requirements, it is necessary that the total lift force remain substantially constant.
Because of the total dependency on the propulsion system alone, the critical periods of VTOL aircraft operation are during vertical takeoff and landing. Of the two, the landing is much more critical because of the exposure time to failures includes the complete duration of the mission. VTOL propulsion system design considerations must therefore include the possibility of one engine core being lost, a condition commonly referred to as "one-core-inoperative" (OCI). This is particularly true in a two-engine aircraft. During such an emergency landing condition, it is necessary to provide a total lift force substantially equal to the aircraft gross landing weight, and also to provide adequate aircraft attitude control. One approach to this situation is to provide a propulsion interconnect system wherein power is transferred from an operable engine core or gas producer to a separate lift augmenter device. Typical lift augmenters include remote gas coupled turbotip fans, gas interconnected ejectors or shaft interconnected fans. Inasmuch as lift augmenters are generally reliable and relatively free of failure, it is only the failures of the gas generators that are normally considered in the one-core-inoperative situation.
Where the lift augmenter comprises a front fan in a turbofan engine, and the respective fans are interconnected by a shaft to accommodate horsepower sharing during normal operation, and especially during a one-engine-out situation, there are additional complications to consider. In such a propulsion system, since the fans are interconnected by a common shaft, they are constrained to operate at identical speeds. However, the total thrust being delivered by the inoperative engine with its core portion shut down is significantly less than the thrust being delivered by the operative engine. Thus, it is necessary to balance out the respective thrust by closing down the inlet guide vanes of the operative engine while opening up those of the inoperative engine. This presents a problem in the operative engine since the core is now required to drive both fans and therefore needs to operate at increased flow and pressure, but a decreasing of the fan flow by closing down of the inlet guide vanes in a conventional full-span IGV tends to reduce the supercharging to the core engine to thereby reduce the core flow because the supercharging pressure is lost. The problem is further complicated by the requirement for thrust modulation to effect aircraft attitude control. This is normally accomplished by opening of the inlet guide vanes one engine to the maximum control position and partially closing the inlet guide vanes on the other engine to the minimum control position. In order to provide this capability, the inlet guide vanes are usually in the partially closed position. That is, the total capacity of the fan system is used only during the maximum control period and at all other conditions the inlet guide vane is partially closed. Thus, during these conditions, and especially when the operative engine is in the minimum control position, the flow of air to the core is reduced because supercharging capability is lost. For example, a fan operating at 100% corrected speed and designed to produce a pressure ratio of 1.50 at a maximum control or zero inlet guide vane setting produces a pressure ratio of 1.39 when the inlet guide vane is closed 15.degree., the setting required to achieve 20% thrust modulation from maximum control to nominal. When this reduction in pressure ratio is coupled with that required to balance the loss of thrust of the inoperative core, the reduction in flow and pressure ratio becomes substantial. As these reductions are applied to the core of the operative engine, considerable supercharging capacity is lost and, as a result, the required size of the core must necessarily be larger in order to accommodate these periods of operation. The core is thus sized by the requirements presented when one core is out during vertical landing, the IGV's of the operative engine are in a minimum control position and the IGV's of the inoperative engine are in a maximum control position.
It is therefore an object of the present invention to provide an improved cross-connected turbofan engine arrangement.
Another object of the present invention is the provision in a VTOL propulsion system for minimizing the size of the core.
Yet another object of the present invention is the provision in a turbofan engine for significantly reducing the bypass flow without reducing the supercharging pressure ratio to the core.
Yet another object of this invention is the provision in a turbofan engine for using substantially the total capacity of the fan hub supercharging during all modes of engine operation.
Another object of the present invention is the provision in a VTOL propulsion system for a thrust varying means which is economical to manufacture and effective in use.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.